The aim of this study is to learn more about the regulation of reproductive hormones in adult men, and specifically to determine whether it is testosterone (the main sex hormone produced by the testis) or estradiol (another sex hormone) which is more important in controlling the secretion of leutinizing hormone (LH) and follicle stimulating hormone (FSH) from the pituitary gland. To do this a medication will be administered which blocks the body's ability to convert testosterone to estradiol. Our hypothesis is that estradiol has a greater inhibitory effect on LH and FSH secretion than testosterone and that consequently blocking the production of estradiol will result in an increase in LH and FSH levels.